Cold work die steel

ABSTRACT

A MALLEABLE COLD WORK DIE STEEL CONTAINING LARGE AMOUNTS OF TITANIUM CARBIDES IN A MATRIX OF TEMPERED MARTENSITE.

April 3, 1973 J. Y. RIEDEL COLD WORK DIE STEEL Filed March 2, 1970 United States Patent O 3,725,050 COLD WORK DIE STEEL `lohn Y. Riedel, Bethlehem, Pa., assignor to Bethlehem Steel Corporation Filed Mar. 2, 1970, Ser. No. 15,482 Int. Cl. C22c 5'9/14 U.S. Cl. 75-126 2 Claims ABSTRACT F THE DISCLOSURE A malleable cold work die steel containing large amounts of titanium carbides in a matrix of tempered martensite.

BACKGROUND OF THE INVENTION This invention relates to tool steels, and more particularly to cold work die steels.

Cold work die steels are commonly used in apparatus for forming sheet steel. In many forming operations, particularly where the deformation is severe, the oxide layer at the surface of the sheet is broken up as the sheet is being deformed. The high pressures accompanying deformation may cause particles of the metal formerly underlying this oxide layer to weld to the die wall. As a result of this welding action, during subsequent deformations one of two things will happen. The Welded particle may be pulled off the die wall along with a small piece of the die itself. This constitutes wear of the die Wall. Alternatively, the welded particle may remain on the die wall and act as a cutting tool on the workpiece. This problem of metal pick-up by the die, referred to in the art as galling, is an important factor in die wear.

Another important factor in die wear is abrasion resulting from small particles of material interposed between the rubbing surfaces of the die and the workpiece.

Resistance to galling and abrasion are not the only factors to be considered in the selection of a cold work die steel. Other factors include toughness, machinability and grindability.

In the past, steels were known which were characterized by the combination of both a fairly good resistance to galling and abrasion as well as good toughness, machinability and grindability. An example of such a steel is as follows:

C Mn

Weight percent 1. O0

Better wear resistance could be obtained by increasing the' amount of carbon and carbide-forming elements in the steel. This however, resulted in a substantial decrease in toughness, machinability and grindability. Examples of such steels are as follows:

Steel C Mn Si Cr V Mo W (A) Weight percent. 1. 50 0.30 0.25 12.00 0.60 0.80 (B) Weight pereent.. 2.4 0. 40 0.40 12.50 4.00 1.00

It is an object of this invention to provide a cold work die steel having the combination of extremely good resistance to galling and abrasion as well as very good toughness, mechania'bility and grindability.

SUMMARY OF THE INVENTION 3,725,50 Patented Apr. 3, 1973 ICC tract from toughness, limiting the chromium content is thus conducive to good toughness.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of a typical prior art cold work die steel.

FIG. 2 is a photomicrograph of an example of the steel of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Broadly, the steels of the invention consist essentially of 0.85 to 1.50 wt. percent carbon, 0.20 to 2.00 wt. percent manganese, 0.80 to 2.00 wt. percent titanium, 3.50 Wt. percent max. chromium, 2.00 wt. percent max. molybdenum, balance essentially iron. In addition, for reasons to be explained hereinafter, the Wt. percent carbon must be equal to at least 0.65 plus 1A times the titanium. The steel may also contain up to 1.20 wt. percent tungsten. By balance essentially iron, I do not wish to exclude residual impurities and incidental elements which may be present in amounts which do not substantially detract from the novel properties of the steel. Preferably, the aluminum content should be limited to 0.10 wt. percent max., the silicon to 0.40 wt. percent max., and the nitrogen to 0.010 wt. percent max.

The reasons for the above range limitations are as follows. Inasmuch as the amount of carbon desired in the steel `depends upon the titanium content thereof, titanium will be discussed rst. The steel should contain a minimum of 0.80 wt. percent titanium to insure that a large amount of titanium carbides form. Increasing the amounts of titanium and carbon in the steel results in an increasing volume percentage of titanium carbides and improved wear resistance. However, the toughness, machinability and grindability decrease as the volume percentage of titanium carbides increases. An excellent combination of these properties can be obtained if the titanium is limited to 2.00 wt. percent max.

The steel should contain a minimum of about 0.55 wt. percent carbon to insure that the as-quenched hardness of the steel is at least 64 Rockwell C. Additional carbon is necessary for combining with the titanium to form titanium carbides. This amount of carbon is equal to, by weight, 1A the amount of titanium. An additional 0.10 Wt. percent carbon has been found necessary to insure a minimum hardness of `60 Rockwell C after tempering the asquenched roll at 300 to 400 F. Thus, the total Wt. percent of carbon in the steel should be equal to at least 0.65 plus it times the titanium. Since the minimum titanium in the steel is 0.80 wt. percent, the minimum carbon is 0.85 wt. percent.

Increasing the volume percentage of carbides in the steel decreases the toughness thereof. For this reason, the carbon content of the steel should be limited to 1.50 wt. percent max.

A minimum of about 0.20 wt. percent manganese is necessary to prevent hot-shortness, while manganese in excess of about 2.00 wt. percent imparts excessive brittleness to the steel. Chromium and molybdenum are included in the steel for the purpose of hardenability. The upper limits th'erefor, set forth above, are necessary to prevent the steel from becoming unduly brittle due to excessive carbide formation.

Tungsten may also be present in the steel. This element substitutes for a portion of the titanium in the titanium carbides, thus increasing the volume percentage of the titanium carbides in the steel. Tungsten, however, also substitutes for a portion of the chromium in the chromium carbides, thus increasing the volume percentage of the chromium carbides in the steel. As above noted, increasing the volume percentage of carbides decreases the 4 toughness of the steel. For this reason, the tungsten must cast into a 9 inch square ingot mold and hot topped. The be limited to about 1.20 wt. percent max. composition of the steel was as follows:

C Mn T1 W Cr Mo Ai si N P S weightpereent 0.99 0.09 0.96 0. s4 3.01 1.05 0.030 0. 35 0.008 0.003 0.007

The aluminum and silicon in the steel are preferably The ingot was heated to about 2050 F., forged into limited to 0.10 wt. percent max. and 0.40 wt. percent a 6 inch square billet, and cooled slowly to room temmax., respectively. The reason for this is that both of perature. The billet was then annealed by a process comthese elements are strong graphitizers. The formation of 10 prising heating to l450 F., holding at said temperature graphite reduces the amount of carbon available for hardfor 10 hours, cooling to 900 F. at a rate of 30 F./hour, ening and also reduces toughness. In addition, silicon conand air cooling to room temperature. The longitudinal tributes to decarburization during heating. surface of the billet was then ground to remove defects Titanium carbide has been found to be much harder therefrom. than titanium nitride. Hence, for a given titanium con- The billet was subsequently reheated to about 2050 F., tent, it is desirable to maximize the amount of titanium forged into a bar 2% x 3% x 36 inches, and cooled slowcarbide and minimize the amount of titanium nitride in ly to room temperature. Next, the bar was annealed by a the steel. Limiting the nitrogen content of the steel to process comprising heating to 1600 F., holding at said 0.010 wt. percent max. results in the formation of a temperature for 8 hours, cooling to 900 F. at a rate of minimum amount of titanium nitrides. 30 F./hour, and air cooling to room temperature. The

For an optimum combination of properties, the followbar was then rough-machined into a die. ing ranges of elements are preferred: The rough-machined die was heat treated by a process C Mn 'ii W or M0 A1 si N wsigutpercent 0.00-i.10 .40-.s0 0. 80-1. 50 0. 90-1. 20 2. 75-3. 25 0. 90-1. 25 0. 01-0. 03 0. 15-0. 40 0. 010

FIGS. l and 2 provide a visual comparison of the microcomprising preheating to l200 F., heating to 1775 F.,

structures of a typical prior art cold work die steel with holding the die at 1775 F. for 1 hour per inch of greatest a steel of the invention. The compositions of these steels cross-section, and quenching by air cooling to room temare as follows: pcrature. The quenched die was then tempered by holding it at 3.50 to 400 F. for 2 hours per inch of greatest Steel C M S1 Cr V M0 W T1 N cross-section. Following tempering, the die was finish- Prior art machined and tested for defects.

ggigiittmm-- 1.02 0.59 0.29 5.2 0. 24 .1.02 Res. 3, The die WaS used in a die for punching sheet steel. The Invention a Service life of the piece was found to be superior to that Sgreclgrllltgnn 1 0() 0.50 0,40 3.() 0.25 1,0 1 00 1.00 0.005 Of Steels COIllCalIllIlg relalVely large amounts Of Chromium and carbon, e.g. steels such as the type above designated Both figures are photomiciographs, at 500 magnicaaS A and B. tions, after a picral etch. In FIG. l, the specimen was 40 IClalmZ austenitized at 1775 F., air quenched, and tempered at 1. A malleable steel consisting essentially of 0.90 to 400 F. for 4 hours. The microstructure consisted of 1-10 Wt Percent Carbon, 0-40 t0 0-80 Wt- Percent man- M23C6- and M703.. type arbide5 in a matrix of temganese, 0.80 to 1.50 wt. percent titanium, 0.90 to 1.20 wt. pel-ed martensite, percent tungsten, 2.75 to 3.25 wt. percent chromium, 0.90 In FIG ,2, the Specimen was austenitized at 1750 F, 45 to1.25 wt. percent molybdenum, balance essentially iron, and oil quenched. The large angular particles are titanium Sald Steel belng adapted t0 be heat treated to have a microcarbides, While the Small rounded particles are M23C6 structure consisting essentially of undissolved carbides in type carbides. The matrix is martensite. a matrix of temperedmartensite. I

It is clear from a comparison of FIGS. 1 and 2 that the 2 A COld Work die formed of the steel of claim 1.

microstructure of the steels of the invention differs substantially from that of the prior art steel shown. The References Cited titanium carbides shown in FIG. 2 are much harderthan UNITED STATES PATENTS both the MaCs-- typ? and the M193* type Cafbldes' 1,839,157 12/1931 Mathesius 75-126 D Said titanium carbideslmpart WeahrjrCSlSal'lCe t0 the Steel' 2,362,046 11/1944 Bonte 75 123 M Steels of the foregoing COmpOSltlOIlS may be Preduced 2,665,205 1/1954 Klaybor 75 126 E in the usual manner. For example, a melt of such a steel 2,132,877 10/1938 Naumann 75 126 C may be prepared in an electric furnace and teemed into 3 330 651 7 /1967 Younkin 75 123 M ingot molds. Following solidication, the ingots are either 1981898 11/1934 Boegehok 75 126 D hot rolled into billets or press forged into other desired 2:923619 2/1960 Culp 11121111 75 -126 C shapes. The resultant prOdllC 1S Subsequently annealed 60 2,968,549 1/1961 Brady 75.. 126 C surface conditioned, and formed into the desired product.

For example, a billet may be hot rolled into bars. The n ,Z

and tempefefl and fmish'gfounl' HYLAND Bizor, Primary Examiner As a specific example of the invention, an experimental 500 lbs. heat of steel was melted in an induction furnace, 

